TAB system LSI packaging, which can cope with a reduction in the interconnection pitch as well as a reduction of the target apparatus in thickness and weight, has been increasingly employed for packaging LSI packages for example in a liquid crystal television, an electronic notebook, a laptop personal computer, a liquid crystal driver IC for a word processor, ICs for a watch and a hand-held calculator, and the like. The term TAB system refers to a method of batch-bonding a plurality of electrodes which are formed on an LSI chip with lead wires provided on a film carrier tape.
FIG. 1 typically illustrates LSI packaging in such a TAB system which is adapted to batch-bond hundreds of electrodes (Au) 2 which are provided on each LSI chip 1 at pitches of 70 to 200 .mu.m with lead wires 4 which are formed on each film carrier tape 3 by etching with a bonding tool 10 which is heated to a temperature of 500.degree. to 600.degree. C. In such bonding, the bonding tool 10 presses the lead wires 4 against the electrodes 2 by a heated head face 10A. According to such a TAB system, it is possible to package a relatively large number of electrodes in a short time as compared with a system employing a wire bonding tool.
FIG. 2 illustrates the shape of a general bonding tool 20 which is employed in the TAB system. In this bonding tool 20, a head portion forming a head face 20A for bonding is supported by a shank 21 which is made essentially of a thermally unexpandable alloy. Such a bonding tool must have a high reliability for withstanding repeated use under a high temperature for a long time, and hence it is necessary to employ a material which is excellent in heat resistance and wear resistance particularly for the head portion.
In order to satisfy such requirement, the head of a conventional bonding tool is made of a synthetic or natural diamond single crystal, a binderless cBN sintered body or a diamond sintered body. However, a natural diamond single crystal has a large performance dispersion due to a large content of impurities, irregular crystal orientation and the like. Further, it is difficult to grow a large-sized single crystal of at least 7 to 8 mm square. A synthesized diamond single crystal, which is one of the materials having a most excellent performance, can be applied to only limited use since the same is extremely high-priced as compared with other materials. On the other hand, a binderless cBN sintered body is inferior in wear resistance to a material which is mainly composed of diamond. Further, a head face of a diamond sintered body cannot uniformly pressurize lead wires under a high temperature due to an extreme warp caused by a binder contained therein. Thus, none of the aforementioned materials is satisfactory for the user.
On the other hand, vapor-deposited polycrystalline diamond can sufficiently provide the required characteristics of diamond since the same contains no binder. Further, it is possible to reliably supply large-sized vapor-deposited diamond at a low cost. Thus, such vapor-deposited diamond has recently been employed as a material which is suitable for a bonding tool. For practical reasons vapor-deposited diamond is prepared by chemical vapor deposition (CVD) of decomposing and exciting a raw material gas which is mainly composed of hydrocarbon such as methane and hydrogen under a low pressure.
Japanese Patent Laying-Open No. 2-224349 (1990) in the name of the present assignee discloses vapor deposition of diamond on a base material of a sintered body essentially made of Si.sub.3 N.sub.4, a sintered body essentially made of SiC, a sintered body essentially made of AlN, or Si, and brazing of the base material provided with such vapor-deposited diamond to a tool substrate essentially made of a metal. In a bonding tool according to this technique, a head portion essentially made of vapor-deposited diamond is connected to the tool substrate through the base material essentially made of the aforementioned sintered body or Si. Further, a head face for bonding is made of a (100) crystal plane and/or a (110) crystal plane of diamond.
Japanese Patent Laying-Open No. 64-5026 (1989) discloses a tool having a shank whose head portion is coated with vapor-deposited diamond. More specifically, the shank is essentially made of a stainless steel material, and diamond is deposited on its head portion by CVD in a thickness of about 5 .mu.m. The foregoing literature discloses that tool warping due to a difference in the thermal expansion coefficients between the stainless steel material and the diamond, if the diamond coating exceeds 10 .mu.m in thickness, remarkably reduces the bonding yield. The literature also discloses that durability of the tool is reduced if the diamond coating is not more than 1 .mu.m in thickness.
In such a bonding tool, the head face is preferably as flat as possible. Immediately after vapor deposition, however, the diamond surface is so coarse that the same must be polished to define the head face. In order to facilitate such polishing, the diamond is so vapor-deposited that the (100) crystal plane and/or the (110) crystal plane thereof is oriented substantially in parallel with the head face surface. However, such a head face formed by the (100) plane and/or the (110) plane cannot be regarded as optimum with regard to the wear resistance of the bonding tool, although this orientation is preferable for the polishing. Thus, a bonding tool having a diamond head face which is flat and hard to wear is needed.
The above first mentioned literature discloses conventional bonding tools with a head portion which is essentially made of diamond having a thickness of 5 to 300 .mu.m. The above further mentioned literature suggests a head portion which is essentially made of a diamond film having a thickness of 1 to 10 .mu.m. The diamond forming such a head portion has a high purity. In general, high-purity diamond has a high rigidity.
Conventional bonding tools include a constant heating type tool and a pulse heating type tool. In the constant heating type tool having a heater which is mounted in a tool substrate, constant heat is transmitted from the heater to a head portion through the tool substrate. In the pulse heating type tool, on the other hand, a substrate consisting essentially of stainless steel, inconel or Mo is heated by instantaneous pulsed energization. The aforementioned conventional diamond bonding tool is suitable for constant heating but rather unsuitable for pulse heating, because a tool having a diamond head has an inferior thermal responsiveness compared to a tool made of a metal, since the diamond head face is heated by heat transmission from the tool substrate. In order to heat the head face of such a conventional diamond tool to a prescribed temperature in response to a current pulse and cool the same, a longer time is required as compared to a tool made of a metal. Thus, it is difficult to reduce a time required for single bonding. Thus, a diamond bonding tool suitable for pulse heating is desirable.
The tool having a shank whose head portion is coated with diamond can be manufactured through a small number of steps at a low cost. However, a vapor-deposited diamond film directly coating such a shank is inferior in adhesion strength to the shank. Therefore, the conventional diamond-coated tool is inferior in durability since the diamond coating is easily separated or cracked during use. Thus, a diamond-coated tool which is superior in durability is desirable.